In the manufacture of semiconductor devices, it has become a standard practice to coat the leads of plastic chip carriers with a thin layer of solder prior to connecting the leads of the chip carrier to a circuit board. This process, commonly referred to as hot solder dipping, is done prior to mounting the chip carriers on the circuit board because it provides an easy and reliable means for soldering the leads to the board. As the carriers are mounted, the solder already present on the leads of the chip carriers is heated such that the solder reflows onto the circuit board connectors, resulting in a connection between the chip carrier and circuit board.
While the solder dipping process may be conducted in a variety of manners, the particular option of interest in the present invention is known as a wave soldering process. In this process, a bi-directional wave of molten solder is generated by a wave soldering machine such as the Hollis TDL-12 manufactured by Hollis Industries. A conveyance mechanism is provided for moving a series of the plastic leaded chip carriers (PLCC) through the molten solder wave such that the molten solder flows onto the leads. To facilitate the handling of a plurality of the chip carriers, pallet fixtures are used such as the Single Unit Plastic Leaded Chip Carrier (PLCC) manufactured by Northwest Precision Stamping Associates (NPSA).
Because of the more difficult environments in which semiconductors are being used such as those in military or space applications, the nonhermetic plastic chip carriers are not appropriate or disallowed. Rather ceramic leadless chip carriers are required for these applications. In addition to the hermiticity of the ceramic carriers, leadless ceramic chip carriers (LCC) can be made to have multiple planes of wiring in the ceramic carrier, resulting in more electrical contact points per unit area of real estate on the chip carrier. This results in a faster and more dense chip carrier unit. As with the leaded chip carriers, the conductive pads on the ceramic leadless chip carriers are hot soldered dipped.
There exist, however, several significant problems with the prior art in providing solder to the conductive pads of the leadless chip carriers. The first problem lies in the coplanarity requirements of the solder once the solder adheres to the pads of the LCC's. Because solder has a high surface tension, it tends to form bumps or beads of varying size when it adheres to the metallic pads of the leadless chip carrier. The coplanarity of the solder is the uniformity of these bumps on a single conductive pad which is determined by measuring the distance between the tangential points of the smallest and the largest bumps. The present automatic system for the PLCC's, when used for the hot solder dipping of the ceramic leadless chip carriers, have not been able to meet the standard industry and military requirements for coplanarity, and thus, nonautomated methods i.e., hand dipping, have been used to apply solder to LCC's.
As a chip carrier is passed by the molten solder wave, the solder will tend to build up on the conductive pad as the conductive pad is moved by the hot solder wave. This is a result of the surface tension of the solder now formed on the conductive pads working against the surface tension of the solder in the solder wave, thereby resulting in solder build up where the break occurs.
The second problem unique to hot solder dipping of LCC's lies in the protecting of the carrier's lid. To achieve the hermeticity of the carrier, a lid, usually formed of gold plated kovar, is sealed to a carrier base, with the carrier base containing a semiconductor device within a cavity formed in the base. One of the industry's requirements for the manufacturing of LCC's is to avoid any contact of solder with the lid, thereby maintaining a clean lid. Additionally, military specifications require that the lid not be bridged to the leads or violate package outline dimensions due to solder on the lid before acceptance of the semiconductor parts made for the military.
As indicated above, the present approach to applying solder to ceramic LCC's such that the above problems are minimized is to hand dip the ceramic leadless chip carriers individually into a bath of molten solder. An operator uses a magnetic fixture to hold and lower the single chip carrier into a flux and then into a static pot such as the Waage Melting Pot containing molten solder. The LCC is carefully lowered into the solder slightly below the surface of the solder and then rocked back and forth such that the LCC's conductive pads only are covered with solder. Although this method does successfully avoid solder from getting onto the lid of the carrier package, this method does not improve upon the coplanarity of the solder bumps to any great extent because of the build up of solder still occurring at the point where the solder on the pad of the chip carrier is detached from the bath of molten solder in the pot. Additionally, this hand dipping method is extremely time consuming and costly to implement as compared to any automated system.
It is, therefore, an object of the present invention to provide a new and improved means for applying solder to ceramic chip carriers.
It is another object of the present invention to provide a new and improved means for minimizing the noncoplanarity of the solder applied to the conductive pads of ceramic leadless chip carriers.
It is still another object of the present invention to provide a new and improved means for applying solder to the conductive pads of the ceramic leadless chip carriers while preventing solder from getting onto the lids of the leadless chip carrier package.
It is further an object of the present invention to provide a new and improved means for carrying leadless chip carriers in an automated wave soldering system.